A power amplification circuit in a wireless system is typically a large signal device. In wireless systems, the power amplifier circuit may transmit output signals at power levels up to 30 dBm, for example. In such wireless systems, which may, for example, utilize modulation types such as binary phase shift keying (BPSK) or 512 level quadrature amplitude modulation (512-QAM), output power levels may vary widely such that the ratio of the peak power level to the average power level may be large, for example, 10 dBm to 15 dBm. Because of these large swings in output power levels, power amplifier (PA) circuits may distort the output signal. Distortion, however, is a characteristic, which may be observed in PA circuits that are utilized across a wide range of applications, and may not be limited to PA circuits utilized in wireless systems. There are two metrics, which may be utilized to evaluate the distortion performance of PA circuits. These metrics may be referred to as amplitude modulation to amplitude modulation (AM-AM) distortion, and amplitude modulation to phase modulation (AM-PM) distortion.
The AM-AM distortion provides a measure of the output power level, Pout, in response to the input power level, Pin. The input power level, and output power level are each typically measured in units of dBm, for example. In an ideal, non-distorting, PA circuit, the output power level changes linearly in response to a change in the input power level. Thus, for each ΔPin change in the input power level there may be a corresponding change in the output power level, ΔPout≈αΔPin, where α represents a constant of linearity. The AM-AM distortion may be observed when, for example, the change in the output power level in response to a first change in input power level may be Δ1Pout≈αΔ1Pin, where the change in the output level in response to a second change in the input power level may be Δ2Pout≈βΔ2Pin, when α≠β.
The AM-PM distortion provides a measure of the phase of the output signal in relation to the input signal (or output phase) in response to the input power level. Output phase is typically measured in units of angular degrees. The AM-PM distortion may be observed when, for example, the output phase changes in response to a change in input power level.
Limitations in the performance of PA circuitry due to distortion may be exacerbated when the PA is integrated in a single integrated circuit (IC) device with other radio frequency (RF) transmitter circuitry such as digital to analog converters (DAC), low pass filters (LPF), mixers, and RF programmable gain amplifiers (RFPGA). Whereas the pressing need to increase the integration of functions performed within a single IC, and attendant increase in the number of semiconductor devices, may push semiconductor fabrication technologies toward increasingly shrinking semiconductor device geometries, these very semiconductor fabrication technologies may impose limitations on the performance of the integrated PA circuitry. For example, utilizing a 65 nm CMOS process may restrict the range of input power levels for which the PA provides linear output power level amplification. Requirements for AM-AM and/or AM-PM distortion levels as set forth in a WLAN standard, such as IEEE 802.11, may preclude transmitting output signals at high output power levels for PA circuitry that is fabricated utilizing a 65 nm CMOS process, for example. An exemplary standard for WLAN systems may specify, for example, that the ratio of the AM-AM distortion to output power level not exceed −25 dBm.
One current approach utilized in an attempt to reduce AM-AM distortion and/or AM-PM distortion involves fabricating PA circuitry in discrete IC devices, which are not integrated with other RF transmitter circuitry. The fabrication processes for these IC devices may utilize gallium arsenide (GaAs) and/or gallium nitride (GaN) materials. Silicon (Si), which is a material utilized in CMOS and various other semiconductor fabrication processes offers several advantages in relation to GaAs and/or GaN. First, silicon is a readily available, and inexpensive, material. Second, Si readily bonds to silicon dioxide (SiO2), a commonly utilized insulating layer during semiconductor manufacturing. Third, the integration of other devices with CMOS circuits is simpler in that many circuits are fabricated on Si and thus would have the same thermal mismatch and other mechanical characteristics.
The discrete IC approach imposes its own limitations, however. One such limitation is that by placing the PA circuitry and other RF transmitter circuitry in discrete IC devices, each device may be required to provide external pins that enable interfacing of the discrete devices. Increasing pin count may increase the manufacturing cost of each IC device. Furthermore, additional external circuitry may be required if the interfaces between the discrete IC devices are not compatible. For example, additional external circuitry may be required if the interface from one discrete IC device utilizes single ended input and output (I/O), while the interface from another discrete IC device utilizes differential I/O.
In another current approach utilized in an attempt to reduce AM-AM distortion and/or AM-PM distortion the input power level may be restricted to a narrower range, which may in turn restrict the maximum output power level. One limitation of this approach in wireless systems is that restricting the maximum output power level may reduce the range over which a system may transmit signals to, for example, a node B element of base transceiver station (BTS) in a wireless network. To the wireless network operator, the reduction in range may require that more node B elements, and/or BTSs be deployed, or risk that wireless network users will experience decreased communications quality, and/or dropped calls when communicating via the network.
Further limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art, through comparison of such systems with the present invention as set forth in the remainder of the present application with reference to the drawings.